The object of such a feeding device is to ensure the supply of a continuously flowing glass strand and to divide it into predetermined fragments (gobs), thus establishing the weight of the individual fragments and thereafter to proceed with the final sectioning. A coupling, mostly electrical, to the subsequently connected glass forming machine synchronizes the movements of the cutting device with the glass forming machine. It is known to activate the drives of the moving mechanical parts of such a feeding, device by compressed air, this, however, has a number of disadvantages, as discussed below.
If, for instance, a movement is to be generated by expanding compressed air, a delay occurs between the actuation of the air release valves and the beginning of the movement, as in the case of a piston in a cylinder. This is a consequence of the compressibility of the air and depends, among others, on the length of the piping, its clear cross section and the prevailing air pressure. A precise control, especially in the case of rapid switch sequences, becomes more and more difficult particularly when large masses are to be moved.
Another very important disadvantage is that a compressed air cylinder as a drive unit can have only an either/or position. For instance, air pressure applied at one end of the piston of the cylinder and presses it with built-up pressure until the piston reaches the opposite end of travel. The same applies to the return travel. It is not possible to stop the piston in a determined intermediate position, since, due to the compressibility of the air, a change in pressure results in a change in the piston travel.
Other mechanisms of the feeding device are driven by points of thrust, determined by a cam disk. At higher speeds the sensor starts to move away from the cam disk, this disturbs both the driving process and the synchronization.